CSEDI: Geochemical Evolution of the Earth's Mantle Constrained by Observations and Dynamical Modeling

点击翻译按钮获取中文摘要

This project seeks to advance scientific knowledge about how the Earth's interior has changed over geologic time, due to the very slow motions caused by the sinking of tectonic plates into the interior and the rise of hot buoyant material from the deep Earth, all of which work to mix the Earth's mantle. These convective motions have the potential to move tectonic plates from the surface all the way to the core-mantle boundary (CMB), and to return material from the CMB where it can contribute to near-surface melting that produces volcanoes at mid-ocean ridges and at oceanic hotspots (such as Hawaii and others). The investigators will use high-performance computer simulations of these convective motions to study the flow of the Earth's interior and how this flow is modified by changes in mineralogy that occur at the high pressures of the Earth's deep interior; the computational models will be enhanced by the inclusion of millions of passive tracer particles that record changes in geochemistry that occur when the mantle melts at the surface, when plates sink and mix into the Earth's interior, and when sediment eroded from the continents is brought into the Earth's interior by these processes. The geochemical changes recorded by the tracer particles will be compared with geochemical data from volcanoes to test whether the flow in the deep Earth continues all the way to the core, or whether the interior flows as separate layers with minimal mixing between them. The knowledge gained from this project will provide information on how the tectonic motions at the surface of the Earth affect mixing in the interior, and how this mixing in turn has altered the flow of various elements from the interior to the continents thus providing information on how the continents have grown and eroded over Earth's geologic history.In this project the investigators will focus on documenting the geochemical evolution of the mantle in a series of high-resolution numerical convection models that examine the degree of mantle layering, stratification, and convective isolation that occur when realistic compressible equations of state are used to describe the physical properties of the mantle. The use of a compressible mantle will allow us to model mineralogical phase transitions in a rigorous way. When combined with laboratory data on the P-T-strain rate dependence of mantle viscosity, conductivity and thermal expansion, these features of the model will produce the most physically realistic description of the extent of mantle layering that can occur due to depth-dependent density and viscosity associated with known phase boundaries. The model series will be further constrained to only those models that match the known ranges in major geophysical and geochemical observations (convective vigor, mantle temperature, heat flow, plate velocities, rate of internal heating, composition and mass of the atmosphere and continental crust, etc.). The geochemistry of the model space will be calculated from millions of active numerical tracers embedded in the mantle flow, that will each record the trace element fractionation and associated isotopic evolution they experience when their evolution is punctuated by melting, degassing, and continental extraction events that occur at the surface of the model. The team will model explicitly the evolving isotope geochemistry of both lithophile and noble gas isotope systems in the convecting mantle, continental crust and atmosphere, and quantitatively compare the model output with geochemical data to test the viability of our models.